Slice the onions, about 3 mm (1/8") thick.
Put the eggs in a little bowl, season with salt and pepper and beat vigorously for few seconds, using a fork.
Add some olive oil into a frying pan (possibly non-stick pan).
When the oil is hot, add the onion slices into the pan and shallow-fry until the bottom side of the slices goes a nice golden colour.
Then, turn the slices and colour the other side. If you have put too much oil, at this stage you can still remove the excess oil with a spoon.
Next, add the eggs into the pan.
Gently shake the pan so that the egg mixture is evenly distributed around the pan and coat all the onion slices. Cook the bottom of the frittata for a couple a minutes or so, then, we need to firm the top side, which still shows some runny eggs.
The best way to firm the top side of the frittata is to put under a broiler for few seconds (40-60 seconds should be enough).
I need the top side slightly dry so that I can turn the frittata by flipping it. Technically speaking, you don’t need to flip the frittata; you can finish it under the broiler or in the oven. but I love flipping it!
One, two, three....the frittata is in the sky.
And here it lands back in the pan topside down.
Cook for another minute, so that you are sure the bottom side of the frittata gets a nice colour. Then, shove it into a plate or onto a greaseproof paper as I did. Slice it and enjoy it.